Field of the Invention
This invention relates to a cordless telephone set, and particularly relates to a digital cordless telephone set which requires a synchronization to digital burst transmission data such as TDM (Time Division Multiplex) data.
Description of the Prior Art
A digital cordless telephone set comprises a base unit which may be connected to a telephone network and a battery-powered handset unit which is connected by a radio frequency (RF) link to the base unit. The RF link is established by sending and receiving a digitally modulated signal between the base unit and the handset unit.
Most digital cordless phones use multiple channels by various multiplexing methods. TDM is one of these methods in which a short transmission data burst is assigned to one of several time frames. TDD (Time Division Duplex) which is also known as Ping-Pong transmission, is an example of TDM which has an advantage of using only one frequency channel for transmitting and receiving. FIG. 1 shows an example of a burst assignment in Ping-Pong transmission between the base unit and the handset unit. As shown in this Figure, the channel is alternately used to transmit and receive respective data signals.
FIG. 2 shows an example of the burst transmission data stream consisting of preamble bits (P), unique word (UW), status bits (S), information data (I), check bits (CHK) and guard bits (G). The preamble bits are used for bit clock recovery, the unique word is used for frame synchronization and the status bits are used for associated control data communication. The information data includes digital voice data and control data, and the check bits are usually used to detect bit errors in the burst frame. The guard bits are empty bits which are used to prevent overlap between the transmitting frame timing and the receiving frame timing.
For an exemplary prior-art cordless phone, the frame synchronization should be established before communication is started. The frame synchronization is established using the unique word.
The receiver first recovers the bit clock from the preamble bits (bit length Lp), the pattern of the preamble bits is, for example, the repetition of "1" and "0" as used in FSK and PSK schemes. The received burst data is sampled in the clock timing determined by the bit clock to output the data.
Next, the receiver locates the unique word (bit length Lu) in the received burst data. Once the unique word is detected, the receiver recognizes that the data after the unique word are the status bits and the information data, and the starting point of the received burst data. That is to say the timing for receiving the burst data is the instant which is (Lp+Lu) bits before the first one of the status bits. The receiver also recognizes that the time for transmitting the burst data is one burst frame length after the beginning of receive timing. Following the scheme, the receiver knows the correct timing to send transmission burst data. In this manner, the frame synchronization is established.
In a normal operation of the digital cordless telephone set, when the transmitter, for example, the base unit, sends burst data at a frequency of F1, the handset unit is going to receive the burst data while scanning frequency channels in order to communicate with the base unit at the frequency of F1.
However, cordless phones are used at various location. Occasionally, the handset unit is located very near to the base unit. In this case, the transmission data may be received at an incorrect frequency because the strength of the signal received at the base station is much greater than during normal operation of the cordless phone. This incorrect reception is one type of spurious reception. Another type of spurious reception is an image reception which occurs in a heterodyne receiving system. Spurious reception may cause problems because, if the cordless phone receives the signal at the incorrect frequency, it will send the transmission data at the incorrect frequency and will never establish the radio link.
For example, if the base unit sends transmission data using TDD at a frequency of F1, and the handset unit receives this data at a frequency of F2 due to spurious reception, the handset will send reply data at the frequency of F2. The base unit is waiting for the reply at the frequency of F1, however, and will newer receive the reply.
Furthermore, such misconnection at a different frequency may interfere with other communications. In the example presented above, this interference would be with another cordless telephone operating at frequency F2.